The present invention relates generally to power amplifiers, more particularly to power amplifiers designed to drive capacitive loads, and, even more particularly, to a transconductance power amplifier.
Many systems, including spacecraft, require power amplifiers to drive capacitive loads. Power amplifier electronics are commonly required in satellites for various servo control systems such as piezoelectric actuators for imaging applications. The high capacitance associated with piezoelectric actuators poses a challenge to maintaining stability and accuracy with conventional amplifier topologies. In addition, high voltage N-channel devices and low voltage operational amplifiers are more readily available in radiation hardened configurations than P-channel devices and high voltage operational amplifiers, respectively. There is thus a longfelt need for a transconductance power amplifier for capacitive loads that offers improved stability and accuracy.
The present invention broadly comprises a transconductance power amplifier for amplifying a signal to a capacitive load, including a first N-channel enhancement MOSFET transistor operatively arranged to source current to the capacitive load, wherein the first N-channel MOSFET transistor has a threshold gate to source voltage, a second N-channel enhancement MOSFET transistor operatively arranged to sink current to the capacitive load, an operational amplifier operatively arranged to transmit and amplify an input signal to both of the first and second MOSFET transistors, and, means for biasing the first N-channel enhancement MOSFET transistor such that its gate to source voltage is always at or above its threshold when the load draws near zero current so that very little additional gate charge is required to turn it on more fully. The power amplifier also includes means for reducing current to the first N-channel enhancement MOSFET transistor when the power amplifier sinks current from the load through the second N-channel enhancement MOSFET transistor.
A general object of the present invention is to provide a transconductance power amplifier for high capacitance loads, which amplifier offers improved stability and accuracy versus prior designs.
A secondary object of the present invention is to provide a transconductance power amplifier for capacitive loads having a pair of N-channel MOSFET drive transistors, one of which is biased such that its gate to source voltage is always at or above its threshold when the load draws near zero current so that very little additional gate charge is required to turn it on more fully.
Another object of the present invention is to provide a transconductance power amplifier for capacitive loads having a pair of N-channel MOSFET drive transistors, one of which sources current to the load while the other sinks current from the load, further including means for reducing current to the xe2x80x9csourcingxe2x80x9d transistor when the other transistor is sinking current from the load.
A further object of the present invention is to provide a transconductance power amplifier which uses N-channel devices and a low voltage operational amplifier for radiation hardening.
Still a further object of the present invention is to provide a transconductance power amplifier which uses a minimum number (5) of active components.
These and other objects, features and advantages of the present invention will become readily apparent from the following detailed description of the invention in view of the drawing figure and appended claims.